Power boring bits

ABSTRACT

A power boring bit is fabricated from a shaft member and a disc which has a channel extending from the circumference thereof to a coaxial aperture through which the shaft extends. The shaft has a pointed tip portion at one end which provides cutting surfaces extending from the point of the tip portion and circumscribing a circle of rotation of maximum radius. A generally axially extending planar shoulder portion is provided on the tip portion with the radial distance to the shoulder portion being less than the maximum radius. One edge portion of the disc bounding the channel is bent to extend from the plane of the body of the disc towards the point and thereby define a cutting lip portion which has a free end providing a cutting edge extending from the periphery of the disc to closely adjacent the axial shoulder portion intermediate the width thereof and inwardly of the circle of rotation of maximum radius.

BACKGROUND OF THE INVENTION

Power boring bits are widely employed for rapid drilling of medium tolarge size holes in wood, plastic and the like. Over the years, varioustechniques have been proposed for assembling a separately formed disc toan elongated shank or bit providing the pointed tip portion.Illustrative of various structures are those shown in Wilson U.S. Pat.Nos. 2,576,376 granted Nov. 27, 1951, 2,577,987 granted Dec. 11, 1951and 2,593,823 granted Apr. 22, 1952; De Liso Pat. No. 2,962,066 grantedNov. 29, 1960; and Mackey Pat. No. 3,165,131 granted Jan. 12, 1965.

It has been recognized that significant economic advantages could beobtained by the use of cutting discs which might be formed from sheetmetal stock by suitable punching, machining and grinding operations.Among the problems that have been encountered in fabricating andassembling such composite power boring bits have been obtaining asatisfactory bond between the two components to prevent relativedisplacement, locating the cutting edge of the disc relative to thecutting edge of the point, providing adequate means for removal of theturnings, and achieving satisfactory transition of the cutting edgeaction from the point to the disc.

It is an object of the present invention to provide a novel power boringbit which is fabricated relatively economically and relatively easilyfrom rod or bar stock for the shaft member and from sheet stock for thedisc member.

It is also an object to provide such a power boring bit wherein the discmember is securely engaged to the shaft member in a manner substantiallyensuring against relative displacement.

Another object is to provide such a power boring bit in which there isachieved smooth transition between the cutting action provided by thepoint and the cutting action provided by the disc.

SUMMARY OF THE INVENTION

It has now been found that the foregoing and related advantages may bereadily attained in a power boring bit which includes a shaft memberhaving an elongated shank portion and a pointed tip portion at one endthereof configured to provide cutting surfaces along a segment of itsaxial length extending from the point of the tip portion. The cuttingsurfaces circumscribe a circle of rotation of maximum radius at alocation along the length of the tip portion which is spaced from thepoint thereof, and the tip portion has a generally axially extendingplanar shoulder portion spaced from the point with the radial distanceto the shoulder portion being less than the maximum radius. Seated onthe tip portion at the axial shoulder portion is a cutter disc having anaperture therethrough and a channel therein extending from the apertureto the circumference thereof. One edge portion of the disc bounding thechannel is bent so as to extend at an angle from the plane of the bodyof the disc toward the point of the tip portion to provide a cutting lipportion and the free end of this cutting lip portion has a cutting edgeextending from the periphery of the disc to closely adjacent the axialshoulder portion of the tip portion intermediate the width thereof andinwardly of the circle of rotation of maximum radius. Suitable means isemployed to bond the disc to said tip portion.

In the preferred aspect, the tip portion is of lesser cross section atits base than the shank portion to provide a radial shoulder thereaboutwith the body of the disc abutting the radial shoulder. The aperturethrough said disc is of D-shaped configuration defined by a rectilinearportion and an arcuate portion extending between the ends of therectilinear portion which is disposed adjacent the axial shoulderportion of the tip portion.

Depending upon the size of the components, the channel in the disc mayextend perpendicularly to a horizontal line across the axial shoulderportion or at an acute angle thereto.

In one embodiment, the tip portion is of polygonal cross sectionthroughout the segment of its axial length and the faces thereofdefining the polygonal cross section taper towards the point. Desirably,the polygonal cross section is provided by a generally pyramidicalconfiguration for the tip portion throughout the segment of its axiallength. In this embodiment, the axial shoulder portion may be providedby a portion of one of the faces of the polygon, and the cuttingsurfaces are defined by the corners of the polygon with the cutting edgeof the cutter disc intersecting one face of the polygon adjacent themidpoint of the width thereof.

In another embodiment, the axial shoulder portion is provided by a flatencompassing only a portion of the periphery of the tip portion, andextending at an angular orientation relative to the axis of the tipportion differing from that of the remainder of the periphery at thelocation of the flat. Here, the tip portion may be of polygonal crosssection throughout the segment of its axial length and the faces thereofdefining the polygonal cross section taper towards the point. The flatthen has an included angle greater than that of the face of the polygonaligned therewith. Using the separate flat is essential when the tipportion has a helical thread providing the cutting surfaces.

The means bonding the disc to the tip portion will normally comprise ametallic deposit such as brazing metal, although other means may be usedsuch as welding, pins, etc.

Most desirably, the cutter disc includes a spur formed at a point aboutthe periphery spaced from the cutting lip portion and extending from theface of the disc disposed towards the point of the tip portion.

In a method for making the power boring bits, an elongated shaft memberis formed with a shank portion and a pointed tip portion at one endthereof with cutting surfaces extending along a segment of the axiallength of the tip portion from the point thereof and circumscribing acircle of rotation of maximum radius at a location along the length ofthe tip portion spaced from the point thereof. The tip portion is formedto provide a generally axially extending planar shoulder portion spacedfrom the point with the radial distance to the axial shoulder portionbeing less than the maximum radius. A disc is formed with a coaxialaperture and a channel extending from its circumference to the aperturewith the portion along one edge of the channel extending at an anglefrom the plane of the body of the disc to provide a cutting lip portion.The disc is then seated on the tip portion of the shaft member with thecutting lip portion projecting in the direction of the point and havingits inner end disposed closely adjacent the axial shoulder portionintermediate the width thereof and inwardly of the circle of rotation ofmaximum radius. Then the disc is bonded to the tip portion.

As indicated hereinbefore, the cutting lip portion may be ground toprovide a cutting edge along the free end thereof extending from theperiphery of the disc to closely adjacent the axial shoulder portion ofthe lip portion. The coaxial aperture in the disc may be formed with arectilinear side edge and an arcuate side edge extending between theends of the rectilinear side edge, and the channel then intersects therectilinear edge intermediate the ends thereof. The disc is assembled tothe tip portion with the rectilinear side edge of the aperture disposedagainst the axial shoulder portion.

The disc may comprise a sheet metal blank and the forming operations areconveniently performed by stamping and punching. The disc may beinitially stamped from sheet metal to provide the coaxial aperture and arib extending from the periphery thereof to the aperture, and a portionof the rib may thereafter be removed to provide the cutting lip portionand a cutting edge thereon.

Most desirably, the tip portion is formed with a base of lesser crosssection than the adjacent portion of the shank so as to provide a radialshoulder thereabout, and the disc is seated firmly against this radialshoulder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view of a power boring bit embodying thepresent invention;

FIG. 2 is a plan view to an enlarged scale of the bit of FIG. 1 showingone embodiment thereof with the cutting lip portion extendingperpendicularly to a horizontal line across said axial shoulder portion;

FIG. 3 is a view similar to FIG. 2 but showing a larger disc embodimentwith the cutting lip portion extending at an acute angle to a horizontalline across said axial shoulder portion;

FIG. 4 is a fragmentary elevational view to an enlarged scale of theside opposite that in FIG. 1;

FIG. 5 is another similar elevational view with the bit rotated 90° in aclockwise direction;

FIG. 6 is a side elevational view of a disc formed in one technique ofmanufacture prior to grinding;

FIG. 7 is a plan view to an enlarged scale of the disc in the embodimentof FIG. 3;

FIG. 8 is a plan view to an enlarged scale of the disc in the embodimentof FIG. 2;

FIG. 9 is a partially exploded view to an enlarged scale of the disc anda fragmentary portion of the shaft of the embodiment of FIGS. 1 and 2;

FIG. 10 shows the disc assembled to the shaft of FIG. 9 and the brazingring prior to assembly thereof;

FIG. 11 is a sectional view along the line 11--11 of FIG. 9;

FIG. 12 is an elevational view of another embodiment of power bore bitusing a helical thread to provide the cutting edges of the tip portion;

FIG. 13 is a plan view of the embodiment of FIG. 12; and

FIG. 14 is a fragmentary elevational view to an enlarged scale of thetip portion of the embodiment of FIG. 12 showing the flat providing theaxial shoulder thereof.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Turning first to FIG. 1 of the drawings, a power boring bit embodyingthe present invention is comprised of an elongated shaft elementgenerally designated by the numeral 10 and a cutter disc elementgenerally designated by the numeral 12 which is mounted upon the shaftelement 10 and secured thereto in a manner which will be described indetail hereinafter.

In the embodiments of FIGS. 1-11, the shaft element 10 has an elongatedshank 14 with a reduced diameter portion 16 at one end thereof which isprovided with flats 15 for seating in the chuck of a drill or brace (notshown). At the other end of the shank 14 is the pointed tip portionportion generally designated by the numeral 18 having a substantiallypyramidical cutting portion defined by three triangular faces 20, 22, 24which taper to a substantial point 26 from the pedestal portion 28. Asseen, the pedestal portion 28 is of generally D-shaped cross sectionwith the projection of the pyramid face 20 providing the straight sideof the D-shaped cross section and thereby forming a generally axiallyextending planar shoulder 30 along the portion thereof adjacent theshank 14 and including the pedestal portion 28. In this embodiment,wherein the cross section is that of an equilateral triangle, the edgesprovided by the intersection of the faces 20, 22, 24 provide the cuttingedges of the tip portion 18. As best seen in FIGS. 9 and 11, thepedestal portion 28 is of lesser radial dimension than the shank 14 soas to provide a radial shoulder 32 thereabout.

Turning now to the cutter disc embodiment of FIGS. 2 and 8, the cutterdisc 12 is conveniently formed by stamping and punching from sheet metalso as to provide a generally planar body 34 with a generally D-shapedaperture 36 coaxially therethrough and a substantially rectilinearchannel 38 extending from the periphery to the rectilinear edge of theaperture 36. In this embodiment, the edges of the channel extendperpendicularly to a horizontal line across the axial shoulder 30 oraxial plane befined by the base thereof. Depending upon dimension of theparts, one edge thereof may extend radially in the pure geometric sense.A portion of the metal of the disc 12 adjacent the channel 38 isdeformed in the direction of the point 26 to provide a cutting lipportion 40 and the free end thereof is ground to provide a cutting edge42. As best seen in FIG. 2, the cutting edge 42 thus extends from theperiphery of the disc 12 and intersects the face 20 of the tip portion18 intermediate the side edges thereof.

Spaced substantially diametrically from the lip portion 40 is a spurportion 44 which projects from the body 34 in the same direction as thecutting lip portion 40 along a chord generally perpendicular to animaginary line defined by an axial plane bisecting the face 20 of thetip portion 18. As best seen in FIG. 1, the free end of the spur portion44 is ground to provide a cutting edge 46.

In the embodiment of FIGS. 3 and 7, the rectilinear channel 38a extendsat an acute angle to a horizontal line drawn across the axial shoulder30 or the axial plane defined by the base thereof. This embodiment isgenerally employed with discs 12 (and tip portions 18) of largerdimensions in order to ensure that the inner end of the cutting edge 42is disposed closely adjacent the axial shoulder 30.

In fabricating and assembling the embodiments of FIGS. 1-11, rod stockis machined or otherwise formed to provide the shaft element 10 with thereduced portion 16 and chucking flats 15 and the pointed tip portion 18with the radial shoulder 32 extending thereabout. Most conveniently, theend providing the pointed tip portion 18 is initially machined toprovide the pedestal portion 28 and radial shoulder 32 thereabout andthen the faces 20, 22, 24 are ground to provide the cutting edges.

In accordance with one technique in part illustrated in FIG. 10, a sheetmetal disc is stamped and punched to provide the D-shaped aperture 36and the rectilinear channel 38 in its desired location. The punching andstamping operation may concurrently deform into the desired orientationthe spur portion 44 and the cutting lip portion 40, or these may beformed as part of separate operations. The cutter disc 12 is thenassembled on the shaft element 10 so that it bears firmly against theradial shoulder 32 with the rectilinear side of the D-shaped aperture 36being disposed adjacent the face 20 and axial shoulder portion 30 of thetip portion 18 as seen in FIGS. 9 and 10. After proper orientation ofthe cutter disc 12 upon the shaft element 10, a ring 48 of brazingmaterial is disposed at the juncture between the disc 12 and tip portion18 and this assembly is heated to cause the brazing compound to flowalong the interfaces thereof and firmly bond the elements in assembly.After heat treatment, the periphery of the disc 12 is groundconcentrically about the shaft element 12, and thereafter the spurportion 44 and cutting lip portion 40 are ground.

In another technique for forming the cutter disc 12, the disc blank mayinitially be stamped to form the spur portion 44 into the configurationseen in FIG. 6. In this manner, grinding time to form the cutting edge46 thereon may be reduced.

Turning now to the embodiment of FIGS. 12-14, the shaft elementgenerally designated by the numeral 110 has a shank 114 and reducedportion 116 with chucking flats 115 as in the embodiment of FIG. 1.However, in this embodiment, the pointed tip portion generallydesignated by the numeral 118 is provided with a helical thread 120 ofgenerally increasing diameter from the point 126 thereof, and the edgesof the helical thread provide the cutting edges of the tip portion 118.As best seen in FIGS. 13 and 14, the tip portion 118 is machined orground to provide a generally axially extending planar shoulder portion130 extending from the pedestal portion 128 towards the point 126.

The cutter disc generally designated by the numeral 112 is formedsimilarly to that of the embodiment of FIGS. 1-11 with the samereference numerals being employed except in the 100 series. It isassembled to the shaft element 110 and secured thereon in a like manner.

In each of the several embodiments, it can be seen that the pointed tipportion 18, 118 defines a circle of rotation having a maximum radiusspaced from its point 26, 126. The cutting lip portion 40, 140 is bentupwardly from the plane of the body 34, 134 of the cutter disc 12, 112and disposed so that the inner end of the cutting edge 42, 142 isclosely adjacent the axial shoulder portion 30, 130 and intermediate theside faces therewith. In the embodiment of FIGS. 1-11, it can be seenthat the three corners of the pyramid shaped tip portion 18 define thecircle of rotation and that the disposition of the inner end of thecutting edge 42 intermediate the width of the face 20 locates itinwardly of the maximum radius circle of rotation of the cutting edges.As a result, the cutting edges defined by the intersections of thepyramidical faces 20, 22, 24 bore an aperture having a radius largerthan the radial distance to the inner end of the cutting edge 42. Thus,the effective portion of the cutting edge 42 begins at a point spacedslightly from the axial shoulder portion 30 and the tendency forclogging and poor cutting action at the juncture of the two distinctcutting edges is avoided.

Similarly, in the embodiment of FIGS. 12-14, the flat providing theaxial shoulder portion 130 permits the cutting edge 142 to extendinwardly of the maximum diameter of the helical thread 120.

In either embodiment, the radial distance to the midpoint of the axialshoulder portion 30 (or the point of alignment with the inner end of thedisc cutting edge 42, 142) is less than the radial distance to thecutting edge of the tip portion 18, 118 at that axial location. As usedherein, the terminology "radial distance to the axial shoulder portion"refers to this radial distance; and the terminology "circle of rotationof maximum radius" refers to the circle circumscribed by the cuttingedges of the pointed tip portion at this axial distance along the lengthof the tip portion.

In operation of the embodiment of FIGS. 1-11, the pyramidical pointedtip portion 18 begins the cutting action and provides a pilot hole ofexpanding diameter as it penetrates into the workpiece. It can be seenthat the corners between faces of the pyramid provide the cutting edges.As the bit penetrates into the workpiece to a depth where the tip 46 ofthe spur 44 contacts the workpiece, it begins to cut the fibers ormaterial of the workpiece, and then the full width of the cut iseffected by the cutting edge 42 of the disc 12 as it rotates about thehole cut by the tip portion 18. Since the inner end of the cutting edge42 is disposed inwardly of the circle of rotation of maximum radius ofthe tip portion 18, it can be seen that there is no discontinuity ofcutting action at the inner end of the cutting edge 46. Moreover, sincethe end is disposed close to the axial shoulder portion 30, there isrelatively little opportunity for chips of material of the workpiece tobecome entrapped therebetween. As the chips of material are cut from theworkpiece, they flow upwardly through the channel 38.

In operation of the embodiment of FIGS. 12-14, the helical thread orauger tip portion 118 similarly provides a pilot hole of expandingdiameter as it penetrates into the workpiece with the chips beingchanneled along the helical thread 120. The inner end of the cuttingedge 142 of the disc 112 extends to a point closely adjacent the axialshoulder 130 and is obviously within the circule of rotation of maximumradius defined by the tip portion 118. As a result, the cutting edge 142cuts highly effectively from the hole provided by the tip portion 118 toits outer end and, as in the case of the previous embodiment, there islittle tendency for chips or turnings to catch in the area between thelip portion 140 and axial shoulder 130.

As will be appreciated from the specifically illustrated embodiments,the tip portion may have either a polygonal configuration or a helicalthread configuration of the auger type. Of the possible polygonalconfigurations wherein the corners defined by the faces of a polygon ofincreasing cross sectional area provide the effective cutting edges, atriangular cross section (and a pyramidical configuration) have provenmost effective. Of the various triangular cross sections which arepossible, an equilateral triangle is preferred because all cutting edgesare effectively cutting.

As is seen from the embodiment of FIGS. 1-11, it is not necessary tomill or machine a separate flat to provide the axial shoulder portionwhen the tip configuration has a polygonal cross section. The desiredplanar surface may simply result from the included angle, or angle oftaper, of the tip portion. Moreover, it will be appreciated that when aflat is employed, it need not extend across the entire width of the faceof that side of the polygon but a groove or other similar area ofsufficient width to permit the cutting lip to slide into position andthe cutting edge to extend inwardly of the circle of maximum rotationmay be effectively employed.

As has been hereinbefore indicated, the disc is conveniently punched andstamped from sheet metal and the same punching operation may punch outthe central aperture and the channel extending from the circumference tothe central aperture. The forming tools may be designed to bend both thespur portion and the cutting lip portion upwardly into the desiredangular orientation. As has been indicated, a rib may be stamped toextend from the circumference to the central aperture, and the channeland the cutting edge may then be formed concurrently by the machining orgrinding operation which removes the undesired metal and forms thecutting edge at the same time. Although the grinding operations formingthe cutting edges on the spur and on the cutting lip portion may beeffected prior to assembly upon the shaft, generally it is desirable toeffect the final grinding operations after assembly to minimize thelikelihood of possible damage to the edges during the assembly andbrazing operations.

As indicated with respect to the two embodiments, the angularorientation of the cutting edge relative to the planar shoulder willvary depending upon the size or diameter of the cutting disc in order tomaintain the inner end of the cutting edge on the disc closely adjacentthe surface of the shoulder. With the smaller size discs requiring arelatively small vertical displacement of the cutting edge from the bodyof the disc, the cutting lip portion may be bent along a line which isperpendicular to a horizontal line drawn across the width of the planarshoulder, and the inner end of the cutting edge will still lie closelyadjacent the planar shoulder, or so close that brazing compound mayeffectively fill the gap therebetween if so desired. As the diameter ofthe disc increases and the amount of vertical displacement of thecutting lip portion accordingly increases, it is desirable to bend thecutting lip portion along an angle to the horizontal line across theface of the planar shoulder. As a result, the inner edge will still belocated closely adjacent the planar shoulder portion on the cutting tip.It is generally preferable to use the perpendicular orientation whenpossible so as to minimize stresses occuring at the juncture between thecutting lip portion and the body of the disc.

The configuration of the central aperture in the cutting disc may varydepending upon the cross section of the tip portion about which it willseat. Although it is possible to use any configuration from circular topolygonal, the preferred configuration is that illustrated whichprovides a straight side to facilitate locating of the disc against theplanar shoulder and an arc extending from the ends of the rectilinearportion which blends smoothly thereinto to eliminate stress points. Byproviding at least one rectilinear segment for the opening, any tendencyfor the disc to rotate about the tip portion is effectively eliminated.By use of the rectilinear side for the aperture, it can be seen that theformation of the cutting lip portion is also facilitated in addition toproviding the means for effecting the desired orientation of the discupon the tip portion of the shank.

To limit the possibility for displacement of the disc relative to theshank and facilitate proper disposition of the disc along the length ofthe tip portion, a radial shoulder is desirably provided on the shank atthe end of the tip portion. In assembly the disc upon the tip portion,the disc will then seat against the radial shoulder and a relativelyextended surface area is provided for bonding between the tip portionand the shank.

Various techniques may be employed for assembling the disc to the shankincluding pinning, welding, adhesives, brazing and the like. Brazing ispreferred since it minimizes the heat required for the assemblyoperation and thereby possible effects upon the properties of the metalsemployed. A ring of brazing material such as shown in FIG. 10 isconveniently utilized with the brazing material flowing about theinterface between the disc and shank to provide a firm bondtherebetween.

From the foregoing detailed specification and attached drawings, it canbe seen that the power boring bit of the present invention may befabricated relatively economically and relatively easily from rod or barstock to provide the shaft member and from sheet stock to provide thedisc member. The configuration of the present invention permits the discmember to be engaged securely to the shaft member in a manner whichsubstantially ensures against relative displacement, and there isachieved a highly smooth transition between the cutting action providedby the tip portion and the cutting action provided by the disc. Themethod of assembly permitted by the present invention affordsversatility in the design and dimension of the components as well aseconomy and facility in fabrication and assembly operations.

Having thus described the invention, I claim:
 1. A power boring bitcomprising:a. a shaft member having an elongated shank portion and apointed tip portion at one end thereof configured to provide cuttingsurfaces along a segment of its axial length extending from the point ofsaid tip portion, said cutting surfaces circumscribing a circle ofrotation of maximum radius at a location along the length of said tipportion spaced from said point thereof, said tip portion having agenerally axially extending planar shoulder portion spaced from saidpoint with the radial distance to said shoulder portion being less thansaid maximum radius, said shoulder portion having a length dimensionextending axially of said shaft member and a width dimension extendingperpendicularly to said length dimension; b. a cutter disc having aplanar body portion and a centrally disposed aperture therethroughreceiving and extending about said tip portion at said axial shoulderportion, said cutter disc having a channel therein extending from thecircumference thereof to said centrally disposed aperture therethrough,one edge portion bounding said channel extending at an angle from saidplane of said body portion of the disc toward said point of said tipportion to provide a cutting lip portion with a root end adjacent saidbody portion of said disc and a free end spaced therefrom towards saidpoint, said free end of said cutting lip portion having a cutting edgeextending from the periphery of said disc to closely adjacent said axialshoulder portion of said tip portion intermediate the width of saidaxial shoulder portion and inwardly of the circle of rotation of maximumradius; and c. means bonding said disc to said tip portion.
 2. The powerboring bit in accordance with claim 1 wherein said tip portion is oflesser cross section at its base than the shank portion to provide aradial shoulder thereabout, and wherein said plane of said body portionof said cutter disc is substantially perpendicular to the axis of saidshaft member, the body portion of said disc abutting said radialshoulder.
 3. The power boring bit in accordance with claim 1 whereinsaid aperture through said disc is defined by a rectilinear portion andan arcuate portion extending between the ends of said rectilinearportion, said rectilinear portion being disposed adjacent said axialshoulder portion.
 4. The power boring bit in accordance with claim 1wherein said channel in said disc extends radially.
 5. The power boringbit in accordance with claim 1 wherein said channel in said disc extendssecantally.
 6. The power boring bit in accordance with claim 1 whereinsaid tip portion is of polygonal cross section throughout said segmentof its axial length and the faces thereof defining said polygonal crosssection taper towards said point.
 7. The power boring bit in accordancewith claim 6 wherein said tip portion is of generally pyramidicalconfiguration throughout said segment of its axial length.
 8. The powerboring bit in accordance with claim 6 wherein said axial shoulderportion is provided by a portion of one of the faces of the polygon andsaid cutting surfaces are defined by the corners of said polygon, saidcutting edge of said cutter disc intersecting one face of the polygonadjacent the midpoint of the width thereof.
 9. The power boring bit inaccordance with claim 1 wherein said axial shoulder portion is providedby a flat encompassing only a portion of the periphery of said tipportion, said flat extending at an angular orientation relative to theaxis of said tip portion differing from that of the remainder of theperiphery at the location of said flat.
 10. The power boring bit inaccordance with claim 9 wherein said tip portion is of polygonal crosssection throughout said segment of its axial length and the facesthereof defining said polygonal cross section taper towards said point,said flat having an included angle greater than that of the face of thepolygon aligned axially therewith.
 11. The power boring bit inaccordance with claim 9 wherein said tip portion has a helical threadproviding said cutting surfaces.
 12. The power boring bit in accordancewith claim 1 wherein said means bonding said disc to said tip portioncomprises a metallic deposit.
 13. The power boring bit in accordancewith claim 1 wherein said cutter disc includes a spur formed at a pointabout the periphery of said disc spaced from said cutting lip portionand extending from the face of said body portion of said disc disposedtowards said point of said tip portion.
 14. The power boring bit inaccordance with claim 1 wherein said plane of said body portion of saiddisc is perpendicular to the axis of said shaft member.
 15. A powerboring bit comprising:a. a shaft member having an elongated shankportion and a pointed tip portion at one end thereof of polygonal crosssection along a segment of its axial length with the faces thereofdefining said polygonal cross section tapering towards the point of saidtip portion, the intersection of said faces of said polygon providingcutting surfaces circumscribing a circle of rotation of maximum radiusat a location along the length of said tip portion spaced from saidpoint thereof, said tip portion having a generally axially extendingplanar shoulder portion spaced from said point with the radial distanceto said shoulder portion being less than said maximum radius, saidshoulder portion portion having a length dimension extending axially ofsaid shaft member and a width dimension extending perpendicularly tosaid length dimension, said tip portion being of lesser cross section atits base than the shank portion to provide a radial shoulder thereaboutand said axial shoulder portion being located adjacent said radialshoulder; b. a cutter disc having a planar body portion and a centrallydisposed aperture therethrough receiving and extending about said tipportion at said axial shoulder portion with the body thereof abuttingsaid radial shoulder, said cutter disc having a channel thereinextending from the circumference thereof to said centrally disposedaperture therethrough, one edge portion bounding said channel extendingat an angle from said plane of said body portion of the disc toward saidpoint of said tip portion to provide a cutting lip portion with a rootend adjacent said body portion of said disc and a free end spacedtherefrom towards said point, said free end of said cutting lip portionhaving a cutting edge extending from the periphery of said disc toclosely adjacent said axial shoulder portion of said tip portionintermediate the width of said axial shoulder portion and inwardly ofthe circle of rotation of maximum radius; and c. means bonding said discto said tip portion.
 16. The power boring bit in accordance with claim15 wherein said tip portion is of lesser cross section at its base thanthe shank portion to provide a radial shoulder thereabout, and whereinsaid plane of said body portion of said cutter disc is substantiallyperpendicular to the axis of said shaft member, the body portion of saiddisc abutting said radial shoulder, said cutting edge of said cutterdisc intersecting said axial shoulder portion adjacent the midpoint ofthe width thereof.
 17. The power boring bit in accordance with claim 16wherein said axial shoulder portion is provided by a portion of one ofthe faces of the polygon.
 18. The power boring bit in accordance withclaim 15 wherein said aperture through said disc portion is defined by arectilinear portion and an arcuate portion extending between the ends ofsaid rectilinear portion, said rectilinear portion being disposedadjacent said axial shoulder portion, and wherein said cutter discincludes a spur formed at a point about the periphery of said discspaced from said cutting lip portion and extending from the face of saidbody portion of said disc disposed towards said point of said tipportion.
 19. The power bore bit in accordance with claim 15 wherein saidtip portion is of generally pyramidical configuration throughout saidsegment of its axial length.
 20. The power boring bit in accordance withclaim 15 wherein said plane of said body portion of said disc isperpendicular to the axis of said shaft member.